As the value of petroleum crude continues to escalate, it will become increasingly more economical to employ enhanced oil-recovery techniques, specially for recovering heavy crude oils. One particularly advantageous enhanced recovery method is the injection of high pressure, substantially pure gaseous carbon dioxide into an oil well. This procedure takes advantage of carbon dioxide's high solubility in crude oil, together with the fact that the viscosity of the crude oil-carbon dioxide solution is significantly lower than the crude oil alone. Consequently, even heavy crude oils can be recovered by injecting the gaseous carbon dioxide into the subterraneous formation in an amount sufficient to saturate the contained oil, followed by the withdrawal of the low viscosity crude oil-carbon dioxide solution from the formation.
A direct consequence of this particular enhanced recovery technique, however, is that the gaseous phase recovered at the well head is contaminated with carbon dioxide, containing as much as 30-90 mol percent carbon dioxide. Since this carbon dioxide disadvantageously reduces the heating value of the recovered natural gas mixture, it must be removed prior to further use of the natural gas. At present, a variety of separation techniques are available for this purpose.
The most prevalent method currently practiced for removing carbon dioxide from gas streams is by a physical or chemical washing or absorption. Solvents commonly used for these procedures include: methanol, amines (e.g., monethanolamine and diethanolamine), propylene carbonate, potassium carbonate and N-methyl-pyrolidone. Unfortunately, with the absorption approach, both equipment size the operating expenses tend to be strongly influenced by the concentration of carbon dioxide in the gas stream to be treated. As the carbon dioxide concentration in the gas stream increase, the costs associated with the use and replacement of the absorption fluid tend to increase significantly. Additional expenses are also incurred for reactivation of the carbon dioxide loaded solvent. Moreover, even though such adsorption systems are designed to minimize thermodynamic inefficiencies, such inefficiencies are generally unavoidable in the regeneration system regardless of whether it operates on pressure differences, temperature differences or some combination thereof.
Adsorption systems have also been used to remove carbon dioxide from gas streams. However, besides being saddled with substantial irreversible energy losses, such systems are also generally limited to the removal of small quantitites of carbon dioxide from gas streams because of economic considerations.
Still another treatment approach employs cryogenic processing techniques. At low carbon dioxide concentrations, advantage is taken of the relatively high freezing point of carbon dioxide relative to the freezing point of other gases with which it is normally found in admixture, by allowing carbon dioxide to selectively freeze out or plate unto heat transfer surfaces; with the subsequent removal therefrom by flowing an essentially carbon dioxide-free gas stream thereover on a subsequent cycle. One skilled in this technology will recognize this as a standard procedure in the air separation arts. However, at higher carbon dioxide concentrations, cryogenic processing techniques have generally been thought to be inapplicable because of the likelihood that the freezing carbon dioxide will plug process piping and equipment, rendering the entire system inoperable. Notwithstanding this potential plugging problem, the potential for a more energy-efficient separation gives the cryogenic technique an inherent advantage relative to the other treatment options. For this reason, the prior art has attempted to define effective cryogenic techniques for rejecting large quantities of carbon dioxide from gas streams.
As early cryogenic approach is illustrated in U.S. Pat. No. 2,632,316--Eastman, which relies solely upon the partial condensation of the carbon-dioxide-containing natural gas for selectively removing carbon dioxide from the gas. As disclosed, the gas mixture is cooled against separated products, including work-expanded carbon dioxide product, at an elevated pressure to partially condense the gas mixture. The bulk of the carbon dioxide is then recovered with the condensed liquid phase; while the gas phase is recovered as the treated fuel gas product. As one skilled in this technology readily recognizes, the quantity and purity of the carbon dioxide removed and the purity of the methane product (treated fuel gas) produced by the Eastman process are determined solely by the temperatures and prevailing pressures during the single condensation and gas-liquid separation steps and will be severely limited by equilibrium mass transfer conditions.
U.S. Pat. No. 3,130,026--Becker, which integrates a step of rectification with the preliminary step of partial condensation, provides an improvement relative to the Eastman process. In the Becker process, as in the Eastman process, the feed gas is cooled against warming product streams including work-expanded carbon dioxide product, at an elevated pressure to partially condense the gas mixture. The non-condensed vapor fraction is then treated in a chemical or physical absorption system to remove its residual carbon dioxide content producing a methane enriched gas; while the condense liquid phase, containing the bulk of the carbon dioxide of the feed gas mixture, is treated to further increase its carbon dioxide concentration. This condensed liquid is initially reduced in pressure, to about 200 psia, for example, and is fed into the top of a rectification zone (stripping column). The liquid is separated into a methane-containing overhead vapor and a carbon-dioxide liquid. The overhead vapor is recycled for further processing with the feed gas mixture, while the liquid is recovered as the carbon dioxide product and is work-expanded to provide process refrigeration. The methane product is recovered from the absorption system at elevated pressure, for example, at about 45 atmospheres absolute; while the carbon dioxide is recovered at substantially atmospheric pressure.
While the Becker process does provide an improvement in the bulk separation of carbon dioxide from the carbon-dioxide-containing natural gas mixture relative to the prior-art Eastman process, it is not without its disadvantages. In the first place, although rectifying the carbon-dioxide-containing liquid stream recovered from the initial separation stage at a low superatmospheric pressure allows the production of a purer carbon-dioxide-containing liquid product, the concentration of carbon dioxide in the overhead vapor of this rectification stage is higher than would be the case at a higher rectification pressure. As a consequence, this overhead vapor cannot economically be treated directly in an absorption system to recover its methane content; instead Becker provides that this stream be recycled to the feed gas mixture. This partially enriched methane-containing vapor must now be reseparated from the feed gas. The energy needed to offset the irreversible energy losses occasioned by this processing method is reflected in the power demands of the recirculating compressor. If possible, such irreversibilities should be avoided. Moreover, rectification at the low superatmospheric pressure also economically inhibits the direct reuse of the separated carbon dioxide product in any enhanced oil recovery operation because of the need for significant recompression of the total carbon dioxide product.
Finally, in terms of the crude methane product produced, the Becker process is no better than the Eastman process since it also relies solely on partial condensation for generating the methane-enriched gas. Accordingly, the purity of the methane product is determined solely by the temperatures and prevailing pressures during the condensation and gas-liquid separation steps, and will be severely limited by equilbrium mass transfer conditions.
Besides the above-mentioned cryogenic processes for the bulk removal of carbon dioxide from gas streams, the prior art also includes cryogenic systems for producing pure products from the gas feed mixture. U.S. Pat. Nos. 4,149,864--Eakman et al and 4,152,129--Drentham et al typical examples. Both patents illustrate single-column rectification systems employing conventional overhead condensers and kettle reboilers. Although useful for producing pure overhead and kettle products, these systems tend to be very energy-intensive as a result of their need to regenerate a sizable methane liquid reflux stream for adequately rectifying the feed gas mixture. As a result, when ultra-high purity products need not be produced, these processes are not generally economically justifiable.